In Silico Prediction of the Melting Points of Ionic Liquids from Thermodynamic Considerations: A Case Study on 67 Salts with a Melting Point Range of 337 °C

Abstract

The melting points (T_fus) of crystalline ionic liquids are calculated from the ratio of the fusion enthalpy and entropy at the melting point where solid and liquid phases are in chemical equilibrium (ΔG^T = 0), and therefore, T_fus = Δ_fusH^T/Δ_fusS^T (if T = T_fus). We specify two variants of this method that have no need for experimental input or tedious simulations but rely on simple calculations feasible with standard quantum chemical program codes and may further be augmented by COSMO-RS. Only single ions are used as input, making the demanding calculation of ion pairs superfluous. The fusion enthalpy is obtained by the principles of volume-based thermodynamics (ion volumes as the major contributor), which may additionally be augmented by COSMO-RS interaction enthalpies for increased accuracy. The calculation of the fusion entropy largely relies on a procedure originally developed for neutral organic molecules that was extended to molecular ionic compounds. Its contributors are the site symmetry σ and the number of torsion angles τ, which are both determined individually for the cation and the anion and are included as their geometric mean. The two methods were tested on several sets of ionic liquids (ILs) and a combination of all sets (67 ILs) that span an experimental melting temperature range of 337 °C. The average error of the simpler, volume-based model (only ion volumes, σ, and τ as input) is 36.4 °C and that of the augmented method (using ion volumes, σ, τ, and COSMO-RS output) is 24.5 °C.